1. Field of the Invention
This invention relates to polycrystalline silicon and in particular to a method by which the diffusivity of impurities in polycrystalline silicon is controlled, and to the resulting structure.
2. Background of the Invention
Polycrystalline silicon is used in monolithic integrated circuits in a variety of ways. Doped polycrystalline silicon is used for contacts to regions formed in the underlying monocrystalline semiconductor material, for conductive interconnection paths on the surface of an integrated circuit and for isolation between active elements of the integrated circuit. One feature of polycrystalline silicon which makes it particularly useful for these applications is that impurities diffuse into it more rapidly than into single-crystal silicon. Recent work shows however that the diffusivity is a strong function of the conditions under which the polycrystalline silicon material is deposited. Tucker and Barry, in patent application Ser. No. 845,822 filed July 29, 1969, now U.S. Pat. No. 3,736,193, disclose a process for electrical isolation in integrated circuits wherein regions of polycrystalline silicon are formed on a doped oxide grid overlying a semiconductor substrate while regions of single-crystal silicon are formed directly on the exposed surface of the semiconductor substrate. Thus it is well-known that polycrystalline silicon can be formed over an oxide layer. Tucker and Barry also disclose that the diffusivity of impurities in polycrystalline silicon is substantially higher than is the diffusivity of the same type of impurities in single-crystal silicon when the polycrystalline silicon is deposited in a narrow range of temperatures centered around 1040.degree. C.